1. Field of the Invention
The present invention relates to a switching regulator of the natural commutation type, and more particularly relates to a switching regulator having a plurality of switching regulators that are arranged in parallel with one another and operated in parallel.
2. Description of the Related Art
A switching regulator increases or reduces power source voltage to supply the voltage-increased or voltage-reduced power to a load. Such switching regulator connects a coil (inductor) to an input power source voltage, increases the current of the coil to accumulate the energy in the coil, then connects the coil to an output terminal to supply the current by means of the accumulated energy, and generates a voltage-increased power in the output terminal. For example, in an in-car switching regulator, the power source voltage of the battery is increased using the switching regulator, and the voltage-increased power is supplied as drive voltage of a load such as an airbag.
In a conventional switching regulator, with regard to hard switching in which switching operation of a switching element is performed while voltage is supplied or current flows, losses from the switching are significant, necessitating the increase of the size of a heat sink, which blocks the reduction of the size of the heat sink.
However, there is proposed a so-called “switching regulator of the natural commutation type” in order to realize soft switching for causing a switching element to perform switching operation in a state where the voltage or current is zero. Such switching regulator is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-282835.
According to this publication, during a period of time when certain control is performed, a coil is connected between a power source voltage and the ground, the current of the coil is then increased to accumulate the energy in the coil, and thereafter the coil is connected to an output terminal to supply the energy accumulated in the coil to the output terminal. At that moment, although the current flowing in the coil decreases in accordance with an LC constant and thereafter gradually commutates into negative current, the switching element is switching at the time when the current flowing in the coil becomes zero.
In this manner, in the switching regulator of the natural commutation type, the current flowing in the coil increases as positive current and then decreases from the peak current to become negative current. Since the voltage-increased energy is different in accordance with the magnitude of the peak current, the peak current needs to be increased more in order to increase the voltage more. However, if the peak current of the coil is increased, the losses of the coil increase as well, necessitating the increase of the coil size.
Therefore, a plurality of switching regulators are provided in parallel so that higher voltage-boosting operation can be performed without increasing the size of each coil, and so that voltage-boosting operation is performed by shifting phases of the switching regulators, whereby current can be prevented from temporarily flowing into the output terminal in large amounts, and generation of noise can be inhibited.
However, control of the switching element for performing soft switching operation of the natural commutation type is complicated. Particularly, it is not easy to perform switching operation in accordance with the output voltage, while shifting the phases of a plurality of switching regulators that are connected in parallel with one another.